Added a warning about #1769 to the routing guide

Conflicts:
	guides/source/upgrading_ruby_on_rails.md

This reverts commit 6df9c595.

This reverts commit 91a1cf70.

Reason: code changes disallowed in docrails, even if they are
guides generation related.

update format validation documetation

For example, you need to change this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end

To this:

  class Author < ActiveRecord::Base
    has_many :posts
    has_many :taggings, :through => :posts, :source => :tagging
  end

  class Post < ActiveRecord::Base
    has_one :tagging
    has_many :taggings
  end

  class Tagging < ActiveRecord::Base
  end

Push add column options to schema creation

Add `DateTime#usec` and `DateTime#nsec` so that `ActiveSupport::TimeWithZone`
keeps sub-second resolution when wrapping a `DateTime` value.

Fixes #10855

removed two instances of add_column_options! from abstract_mysql_adapter

reworked rename_column_sql to remove add_column_options from schema_statements

changed to use new hash syntax.

Refactor of ::Rails module

Use sprockets-rails 2.0.0

Drop extra variable from test

Fix typos in AR changelog [ci skip]

Warning removed for Minitest

Grammar fix [ci skip]

1. Used ActiveSupport::Autoload to dry-up the autoload definitions.
2. Used ActiveSupport's delegate to dry up the application proxied
   attributes.
3. Did a little white space change on Rails.groups.